Referring to FIG. 1, when a transmitting apparatus 102 transmits a signal wirelessly over a channel h(t) to a receiving apparatus 104, the signal becomes spread out in time due to the effects of the channel. As will be familiar to a person skilled in the art, this is typically due in large part to the signal propagating via multiple paths of differing length, e.g. due to the signal bouncing via buildings or geographical features. The channel h(t) is sometimes understood by considering what would happen if an infinitesimally narrow unit impulse (delta function) was input into the channel by the transmitter 102. In that case, the signal as initially received at the receiving apparatus 104 would have its energy spread out over time according to some function h(t) depending on the channel in question. This effect is shown schematically in FIG. 2. If not corrected for, this will inevitably have a deleterious effect on the ability to receive meaningful data.
To address this problem, the receiving apparatus comprises an equaliser 106 coupled to a receive antenna via a suitable radio frequency (RF) front-end (not shown). The equaliser 106 is in effect a filter designed to attempt to apply an inverse h−1(t) to the received signal and thereby remove or at least mitigate the effect of the channel. Of course in a real digital system the inverse of the channel has to be approximated using a finite number of discrete filter coefficients to process a finite number of discrete samples. That is:
      y    k    =            ∑              ℓ        =        0            L        ⁢                  w        ℓ            ⁢              r                  k          -          ℓ                    where k is an integer index denoting the sample number currently being output by the equaliser, yk is the corresponding output for the kth output sample, l=0 . . . L are integer indices denoting nearby received samples which contribute to yk due to the non-instantaneous nature of the channel, rk−1 is the corresponding input sample, and w1 is the corresponding weight quantifying the amount of the contribution from the respective received sample. The sample indices correspond to certain unit time intervals, e.g. chips of a CDMA system or fractions of chips if over-sampled. The received samples l=0 . . . L together represent a certain window of samples over which the received contribution from the channel is considered non-negligible, i.e. for any given output the equaliser only processes the contribution from input samples within the time window L (potentially the effect of the channel stretches away indefinitely in time but beyond a certain point becomes negligible). In this case the window in question may sometimes be referred to as the equalisation length.
One way in which the performance of an equaliser can be improved is to provide an adaptive equalisation length or other such adaptive window which is varied in dependence on channel conditions. Existing applications that disclose this are WO 2009/056499 and WO 2009/121795.
These vary the equaliser length by reducing (or increasing) the number of weight coefficients w used in the final filter, or equivalently the number L of received samples r over which the equalisation is performed. This amounts to varying the length of the filter estimating the inverse of the channel.
It is also possible to perform at least part of the equalisation in the frequency domain by applying a discrete Fourier transform, in which case an alternative way of varying the equaliser length may involve varying the number of frequency domain coefficients.
Another way of adapting a window used in an equaliser is to vary the channel length over which the channel is initially estimated for the purpose of calculating the w coefficients used in the equaliser filter (e.g. see WO 2009/121795, page 18, lines 12-31). That is, rather than (or in addition to) varying the number of coefficients w used to process samples in the filter, it is alternatively or additionally possible to vary the length over which the channel is considered significant for the purpose of calculating the w vector in the first place (i.e. for calculating the inverse of the channel). This window is also a feature of the equaliser and also has an effect on the complexity and/or power. The length of this equalisation window can also be adapted dynamically based on channel conditions.